cmd/internal/obj/arm64: factor out splitting of 24 bit unsigned scaled immediates
Rather than duplicating this code, factor it out into a function and
add test coverage.
Change-Id: I37ce568ded4659d98a4ff1361520c5fb2207e947
Reviewed-on: https://go-review.googlesource.com/c/go/+/512537
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>